1. Field of the Invention
This invention concerns an information recording medium for use in optical disks.
2. Description of Related Art
For recording information on thin films (recording films) by irradiation of a laser beam, various principles have been known. Among them, those utilizing the change of the arrangement of atoms by the irradiation of a laser beam such as phase change of the film material (also referred to as phase transfer and phase transformation) scarcely cause deformation of the thin films and, accordingly, have an advantage capable of obtaining an information recording medium of a both side disk structure by directly bonding two sheets of disk members.
Usually, the information recording medium described above has a constitution comprising a protective layer, a recording film such as made of a GeSbTe series material, a protective layer and a reflective layer formed on a substrate.
In this specification, the term xe2x80x9cphase changexe2x80x9d includes not only the phase change between a crystalline state and an amorphous state but also includes phase change between melting (change to liquid phase) and recrystallization, as well as phase change between a crystalline state and another crystalline state. Further, xe2x80x9cmark-edge recordingxe2x80x9d means a recording system of corresponding an edge area of a recording mark to signal xe2x80x9c1xe2x80x9d and inter mark and intra mark areas to signal xe2x80x9c0xe2x80x9d. In this specification, the optical disk means a disk containing information that can be regenerated by the irradiation of light and/or a device for regenerating information by the irradiation of light.
In a rewritable optical disk such as DVD-RAM, a recording track comprises a pre-formatted area in which address pits are disposed and a user-data area having a tracking groove for recording and conducts information writing or reading after confirming the address and detecting clocks or synchronization signals.
However, since deformation formed by stresses exerting between stacked films and a substrate is different between the pre-formatted area and the user-data area, the recording track is bent relative to the pre-formatted area. This may cause a situation the address data in the pre-formatted area can not be read, assuming a case of push-pull tracking relative to the tracking groove, at a high recording track density with a recording track width of 0.62 xcexcm or less for an optical spot diameter represented by xcex/NA of about 0.96 xcexcm. This may also cause offset in a recording area to partially erase data of adjacent tracks when tracking offset is corrected so as to situate at a normal position relative to the pre-formatted area.
It is considered that the deformation is different between the pre-formatted area in which address pitch are disposed and the user-data area for recording because the user-data area has tracking grooves and the inclined portion of the grooves undergoes force and tends to deform. Further, as another problem caused by the stresses exerting between the stacked films and the substrate, when a number of tracks are rewritten by overwriting for multi-cycles, the substrate of the surface tends to thermally expand and deform by during recording and the tracking grooves are bent in the direction of undergoing the force by the stress exerting from the stacked films to the substrate. Bending occurs remarkably toward the vicinity of the center for the multi-time recording area for 500 cycles or more.
In view of the above, this invention intends to overcome such problems and provide an information recording medium with no deformation of tracking grooves caused by stresses between stacked films and a substrate upon high density writing/reading, capable of possessing favorable writing/reading characteristics, having large process margin, capable of using a manufacturing apparatus at a reduced cost, excellent in view of material cost and mass productivity and with less stresses.
For overcoming the foregoing problems, the information recording medium according to this invention adopts the following countermeasures. That is, the temperature of the substrate is kept as low as possible. When the temperature of the substrate rises during film preparation and the film is deposited in an expanded state and then cooled, the stress from the substrate to the films changes in the direction of increasing the compressive stress. This invention intends to prevent occurrence of stresses by keeping the temperature of the substrate from rising.
Specifically, this invention provides multi-time rewritable information recording media as described below.
(1) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film, an upper protective layer and a reflective layer from the light-incident side, with the interval between tracks being 0.62 xcexcm or less.
(2) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film, an upper protective layer and a reflective layer from the light-incident side, in which
90 atomic % or more for the material of the reflective layer comprises any one of Cr, Crxe2x80x94Al, Crxe2x80x94Ag, Crxe2x80x94Au, Crxe2x80x94Ge, or a Cr alloy as a main ingredient, an Al alloy such as Alxe2x80x94Ti, Alxe2x80x94Cr, Alxe2x80x94Co as a main ingredient, or Gexe2x80x94Cr, Gexe2x80x94Si, Gexe2x80x94N, Co, Ni, Mo, Pt, W, Ge, Sb, Bi, Ag, Au or Cu.
(3) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film, an upper protective layer and a reflective layer from the light-incident side, in which
90 atomic % or more for the material of the lower protective layer comprises any one of oxides of:
ZnSxe2x80x94SiO2, ZnSxe2x80x94Al2O3, ZnSxe2x80x94Ta2O5, ZnSxe2x80x94SnO2, ZnSxe2x80x94In2O3, ZnSxe2x80x94TiO2, ZnSxe2x80x94Cr2O3, ZnSxe2x80x94ZnO or ZnO, SiO2, Al2O3, Ta2O5, SnO2, In2O3, TiO2, SnO2xe2x80x94In2O3, Cr2O3, ZnOxe2x80x94SiO2, Al2O3xe2x80x94SiO2, Ta2O5xe2x80x94SiO2, SnO2xe2x80x94SiO2, In2O3xe2x80x94SiO2, TiO2xe2x80x94SiO2, SnO2xe2x80x94In2O3xe2x80x94SiO2, Cr2O3xe2x80x94SiO2, ZnOxe2x80x94Al2O3, Ta2O5xe2x80x94Al2O3, SnO2xe2x80x94Al2O3, In2O3xe2x80x94Al2O3, TiO2xe2x80x94Al2O3, SnO2xe2x80x94In2O3xe2x80x94Al2O3 and Cr2O3xe2x80x94Al2O3,
a mixture of the materials described above and the material formed by partially or entirely substituting the material described above with a nitride.
(4) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film; an upper protective layer and a reflective layer from the light-incident side, in which
90 atomic % or more for the material of the lower protective layer comprises any one of ZnOxe2x80x94In2O3, SnO2, SnO2xe2x80x94In2O3, ZnOxe2x80x94SiO2, SnO2xe2x80x94In2O3xe2x80x94SiO2.
(5) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film, an upper protective layer and a reflective layer from the light-incident side, in which
90 atomic % or more for the material of the upper protective layer comprises any one of oxides of:
ZnSxe2x80x94SiO2, ZnSxe2x80x94Al2O3, ZnSxe2x80x94Ta2O5, ZnSxe2x80x94SnO2, ZnSxe2x80x94In2O3, ZnSxe2x80x94TiO2, ZnSxe2x80x94Cr2O3, ZnSxe2x80x94ZnO or ZnO, SiO2, Al2O3, Ta2O5, SnO2, In2O3, TiO2, SnO2xe2x80x94In2O3, Cr2O3, ZnOxe2x80x94SiO2, Al2O3xe2x80x94SiO2, Ta2O5xe2x80x94SiO2, SnO2xe2x80x94SiO2, In2O3xe2x80x94SiO2, TiO2xe2x80x94SiO2, SnO2xe2x80x94In2O3xe2x80x94SiO2, Cr2O3xe2x80x94SiO2, ZnOxe2x80x94Al2O3, Ta2O5xe2x80x94Al2O3, SnO2xe2x80x94Al2O3, In2O3xe2x80x94Al2O3, TiO2xe2x80x94Al2O3, SnO2xe2x80x94In2O3xe2x80x94Al2O3 and Cr2O3xe2x80x94Al2O3, a mixture of the materials described above and the material formed by partially or entirely substituting the material described above with a nitride.
(6) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film, an upper protective layer and a reflective layer from the light-incident side, in which
90 atomic % or more for the material of the upper protective layer comprises any one of a mixed material comprising ZnSxe2x80x94SiO2, ZnSxe2x80x94Al2O3, ZnSxe2x80x94Ta2O5, ZnSxe2x80x94SnO2, ZnSxe2x80x94In2O3, ZnSxe2x80x94TiO2, ZnSxe2x80x94Cr2O3, ZnSxe2x80x94SiO2, ZnSxe2x80x94Al2O3, Znsxe2x80x94Ta2O5 and ZnSxe2x80x94Cr2O3 
in which the compositional ratio of ZnS is 60 mol % to 90, or a mixed material comprising ZnSxe2x80x94SnO2, ZnSxe2x80x94In2O3. ZnSxe2x80x94TiO2 and ZnSxe2x80x94ZnO in which the compositional ratio of ZnS is 50 mol % to 85 mol %.
(7) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film, an upper protective layer and a reflective layer from the light-incident side, in which
95 atomic % or more for the material of the recording film comprises Gexe2x80x94Sbxe2x80x94Te.
(8) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film, an upper protective layer and a reflective layer from the light-incident side, in which
the thickness of the recording film is 7 nm or more and 13 nm or less.
(9) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film, an upper protective layer, a reflective layer and at least one interface layer from the light-incident side, in which
95 atomic % or more for the material of the at least one interface layer comprises any one of Cr2O3, Crxe2x80x94N, Gexe2x80x94N, Snxe2x80x94N, Gexe2x80x94O or a mixture of such materials; SiO2, Al2O3, Ta2O5, or a mixture of Ta2O5 and Cr2O3 or Crxe2x80x94N, Gexe2x80x94N, or Gexe2x80x94O; ZrO2, Y2O3, Cr2O3, or a mixture of CrN, GeN, Ta2O5; CoO, Cr2O, NiO, AlN, BN, CrN, GeN, HfN, Si3N4, Alxe2x80x94Sixe2x80x94N series material; Sixe2x80x94N series material; Sixe2x80x94Oxe2x80x94N series material; and nitrides such as TaN, TiN, and ZrN.
(10) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film, an upper protective layer, a reflective layer and at least one interface layer from the light-incident side, in which
95 atomic % or more for the material of the at least one interface layer comprises Cr2O3.
(11) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film, an upper protective layer, a reflective layer and at least one interface layer from the light-incident side, in which
95 atomic % or more for the material of the at least one interface layer comprises Snxe2x80x94N.
(12) A multi-time rewritable information recording medium conducting recording by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 to 60 nm, a recording film, an upper protective layer, and a reflective layer from the light-incident side, in which
the total for the thickness of the entire stacked films containing each of the layers described above is 150 nm or less.
(13) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 to 60 nm, a recording film, an upper protective layer, and a reflective layer from the light-incident side, in which
the reflectance in a crystalline state at each wavelength as viewed on the side of the substrate is higher than the reflectance in an amorphous state at each wavelength in the entire wavelength in a range of 500 nm to 600 nm.
(14) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 to 60 nm, a recording film, an upper protective layer, and a reflective layer from the light-incident side, in which
the transmittance in an amorphous state and in a crystalline state is about 2% or more at least at one wavelength in a range from 610 to 710 nm.
(15) An information recording medium as defined in any one of (2)-(14) above wherein the distance between the tracks of the medium is 0.62 xcexcm or less.
(16) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm, a recording film, an upper protective layer, a reflective layer, and an interface layer in which 95 atomic % or more in the composition comprises SnO2 or Snxe2x80x94Oxe2x80x94N from the light-incident side.
(17) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate of a thickness of 0.7 mm or less, a lower protective layer of a thickness of 20 nm to 60 nm or less, a recording film, an upper protective layer, in which 95 atomic % or in the composition comprises SnO2 or Snxe2x80x94Oxe2x80x94N, and a reflective layer.
(18) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate, a stacked films including,
a lower protective layer with a thickness of 20 nm to 60 nm, a recording film, an upper protective layer and a reflective layer from the light-incident side, and
an adhesive layer from the light-incident side, in which
the distance from the surface of the substrate to the adhesive layer is 150 nm or less, the thickness for each of the stacked films is 40 nm or less and the interval between the tracks is 0.54 xcexcm to 0.62 xcexcm.
(19) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate, a stacked films including,
a lower protective layer with a thickness of 20 nm to 40 nm, a recording film, an upper protective layer and a reflective layer from the light-incident side, and
A an adhesive layer from the light-incident side, in which
the reflective layer is two or more reflective layers comprising 80 atomic % or more of a metal,
the distance from the surface of the substrate to the adhesive layer is 150 nm or less, the thickness of the film in each of the stacked films is 40 nm or less and
the distance between the tracks is 0.54 xcexcm to 0.62 xcexcm.
(20) A multi-time rewritable information recording medium conducting writing by the change of arrangement of atoms under the irradiation of light, comprising a substrate, a stacked films including,
a lower protective layer with a thickness of 20 nm to 60 nm, a recording film, an upper protective layer and a reflective layer from the light-incident side, and
an adhesive layer from the light-incident side, in which
the distance from the surface of the substrate to the adhesive layer is 90 nm to 150 nm, and
the distance between the tracks is 0.54 xcexcm to 0.62 xcexcm.
It is also important to define the total thickness of the stacked films to 150 nm or less for preventing occurrence of stresses.
When the thickness for the lower protective layer or other layers is reduced, it requires a countermeasure for optically ensuring reproduced signals, reflectance, and absorptivity in a optimal range, or a countermeasure for thermally preventing undesired effects of expansion due to the temperature elevation on the surface of the substrate and preventing recrystallization at the periphery of the recording mark, erase of adjacent tracks and erase by the reading light due to the change of heat diffusion.
The method of measuring the groove deformation caused by the stress is as described below.
The method of measuring the stress deformation amount of groove is to be explained. This is explained here in details only to a case of measuring the stress deformation amount of groove in the grooves as an example.
A disk to be measured is set to an evaluation tester and rotated. Then, an optical head is moved to the vicinity of a track for which the stress deformation amount of groove is to be measured. Auto-focusing is applied at that position and a tracking area signal (differential signal) is monitored by an oscilloscope. Then, the gain of the auto-focus is controlled such that the tracking error signal amplitude becomes maximum (AF offset control). Then, tracking is applied to the groove in a state of applying the auto-focusing. Then, writing is conducted while changing the laser power by random signals, a recording power to make the displacement (asymmetry) of the center line for an envelope of signals corresponding to 3T (shortest) mark and space from the center line for an envelope of signals corresponding to long mark and space to 5%, is determined and defined as an optimal recording power. Then, a relation between the radial (radial direction) tilt and a jitter value after 10 cycles of overwriting (optimal power) is measured by a time interval analyzer (TIA) to determine a radial tilt to minimize the jitter. That is, while varying the radial tilt, the current jitter is measured and the radial tilt to minimize the jitter is determined and defined as an optimal radial tilt. Then, tracking offset control is conducted. At first, overwriting is conducted for ten cycles at the optimal power to the lands on both sides of the groove. Subsequently, cross talk from the land in the groove is measured by a spectral analyzer. The tracking gain is controlled so as to minimize the cross talk. Then, it is further preferred to subsequently determine the optimal radial again and, further, applying tracking offset control.
Finally, after completing the AF offset control, tracking offset control and radial tilt control in the groove, the beam is moved to a track for measuring the stress deformation amount of groove. A reproduced signal for an ID area (area expressing address information or the like with pit) arranged being displaced each by xc2xd track on the right and left of the track is measured (added signal) is monitored, and voltage amplitude V1 and voltage amplitude V3 of ID1 and ID3 are measured respectively. Based on the values, |(V1xe2x88x92V3)/(V1+V3)| is calculated.
In the same manner, the stress deformation amount of groove in the land is measured.
This invention provides an advantageous effect in a case of a recording density (track pitch, bit pitch) higher than 2.6 GB DVD-RAM standards and, particularly, in a case of recording density of higher than 4.7 GB DVD-RAM standards. Where the wavelength of an optical source is not at the vicinity of 660 nm or the numerical aperture (NA) of a condensing lens is not 0.6, the invention provides an advantageous effect at a recording density, in which they are converted both in the radial direction and the circumferential direction by wavelength ratio and NA ratio.
The basic technique in the recording device using the phase change recording medium according to this invention (optical disk drive) is as described below.
(1 Beam Overwrite)
The phase change recording medium is usually rewritten by overwriting (information rewriting by overwriting with no previous erase). FIG. 2 shows the principle. When a recording film is melt by a high laser power, a recording mark in an amorphous state is formed by quenching after irradiation irrespective whether the previous state is a crystalline state or an amorphous state. When the recording film is heated to a temperature below a melting point at which the crystallization velocity is high by an intermediate laser power, an area which was previously in an amorphous state is changed into a crystallization state. An area originally in a crystalline state remains as crystalline. Since it is considered that DVD-RAH often records moving pictures, it records long information all at once. In this case, recording after previously erasing the entire information takes twice time and it may requires an enormous capacity of buffer memory. Accordingly, overwritability is an essential condition.
(Mark Edge Recording)
DVD-RAM and DVD-RW adopt a mark edge recording system capable of attaining high density recording. The mark edge recording corresponds the positions at both ends of a recording mark formed on a recording film to digital data xe2x80x9c1xe2x80x9d. This can increase the density by corresponding the length of the shortest recording mark to 2-3 instead of 1 of the reference clock. DVD-RAM adopts a 8-16 modulation system and corresponds to three reference clocks. As shown in FIG. 3 by comparison, it has an advantage capable of high density recording without extremely reducing the recording mark compared with a mark position recording of corresponding the central position of the circular recording mark to digital data xe2x80x9c1xe2x80x9d. However, it is required for a recording medium that the shape deformation of the recording mark is extremely small.
(Format)
As an arrangement for the header area at the beginning of such sector shown in FIG. 4, since DVD-RAM has a format of dividing one circumference into 24 sectors, it enables random access recording. Thus, it can be used in a wide application use including personal computer incorporated memory devices, as far as DVD video cameras and DVD video recorders.
(Land/Groove Recording)
As shown in FIG. 5, DVD-RAM decreased cross talk by land/groove recording of recording both inside of the tracking grooves and convex portion between the grooves. Since the land/groove recording utilizes a phenomenon that the recording marks in adjacent tracks become less visible both in the land and in the groove when the groove depth is defined near xcex/6n (xcex: laser wavelength, n: substrate refractive index) relative to bright and dark (dense and thin) recording mark, the track pitch can be narrowed as 0.615 xcexcm in an example of 4.7 GB DVD-RAM. It is required for the phase difference between the recording mark and other portions than described above, that is, the phase differential component of the reproduced signal is designed such that it exerts in the direction tending to cause cross talk and reduce the same sufficiently. Since the phase differential components of the reproduced signal are added in an inverse phase to the dense/thin reproduced signals of the land and the groove, it also causes unbalance in the reproduced levels between the land and the groove.
(ZCLV Recording System)
In the phase change recording medium, when the recording waveform is not changed, it is desirable to record at an optimum linear velocity corresponding to the crystallization rate for obtaining favorable writing/reading characteristics. However, upon access between recording tracks of different radii on the disk, it takes much time for changing the number of rotation for making the linear velocity identical. In view of the above, as shown in FIG. 6, DVD-RAM adopts a ZCLV (Zoned Constant Linear Velocity) system of dividing the radial direction of a disk into 24 zones and making the number of rotation constant in one zone and change the number of rotation only when other zone is to be accessed. In this system, since the linear velocity is somewhat different between the inner most circumferential track and the outermost circumferential track in the zone, the recording density also differs somewhat but recording can be done substantially at the maximum density over the entire disk region.
The technique for the recording medium according to this invention is as described below.
(Absorptivity Adjustment)
In a high linear velocity (8.2 m/s) medium such as a 4.7 GB/face medium, since previous erase that can be expected in a low linear velocity (6 m/s) medium such as 2.6 GB/face DVD-RAM of DVD-RAM (a phenomenon in which a recording mark is previously erased in a band-like area within a temperature range from 300xc2x0 C. to 500xc2x0 C. ahead of the area in which the recording film is melted by beam spot irradiation) can no more be expected sufficiently, it is essential to keep the absorptivity ratio Ac/Aa between inside and outside of the recording mark to 0.8 or more. By the absorptivity adjustment, the edge portion of the mark can be recorded exactly as shown in FIG. 7. The absorptivity adjustment includes a method of reducing the thickness of the reflective layer to transmit a light such that light absorption to the recording film is not increased in a recording mark portion at a low reflectance (Noboru Yamada, Nobuo Akahira, Kenichi Nichiuchi, and Keisho Fukukawa: High Speed Overwrite Phase Change Optical Disk: Electronic Information Communication Society, Technical Study Report MR 92-71, CPM 92-148 (1992) 37). Cr, Al and an alloy containing one of them is used for the reflective layer in order for the absorptivity ratio adjustment and for keeping the contrast high. The layer properly absorbs light while properly transmits light such that light transmitting the recording film at a recording mark portion of low reflectance is reflected at the reflective layer and is again absorbed in the recording film to prevent temperature from rising excessively and adjust the ratio Ac/Aa to 1 or greater.
In a high density phase change optical disk, since the track pitch is narrow, it is necessary to take a consideration for the phenomenon referred to as a cross erase in which a portion of the recording mark already written in adjacent tracks is erased. In order to prevent cross erase, the heat diffusion in the vertical direction is important. One of the reason for this is that heat less conducts in the direction of the adjacent tracks by vertical diffusion. When Ac/Aa is greater than 1, temperature elevation is reduced in the recording mark area of adjacent tracks and this serves positively in view of prevention of cross erase.
For preventing the cross erase, prevention of recrystallization is also important. As shown in FIG. 8, in a case where a portion remaining as an amorphous recording mark is narrowed by recrystallization from the periphery after the melting of the recording film upon recording, it is necessary to melt a more wide area for forming a recording mark of a predetermined size, which tends to rise the temperature of adjacent tracks. When heat dissipates in the vertical direction, this can prevent recrystallization. This can also prevent heat in the central portion from dissipating laterally during formation of the recording mark to retard cooling in the periphery of the molten area and facilitate crystallization.
(Lower Protective Layer)
The lower protective layer is a stacked film of a thickness of 20 nm or more disposed between a substrate and a recording film for protecting the recording film. For suppressing the groove deformation of the substrate, it is necessary to restrict the thickness of the lower protective layer so that the substrate temperature does not rise during film preparation. In view of the above, the lower protective layer is disposed to a thickness of 20 nm to 60 nm. For increasing the contrast optically, the refractive index is preferably 1.4 to 1.9. However, since the material of low refractive index generally has slow sputtering rate and, accordingly, the refractive index is preferably from 1.6 to 1.9 in view of the mass productivity. The extinction coefficient k is preferably approximate to 0 as much as possible. Further, since the thermal conductivity of the layer is higher than that of the recording film by one digit or more, the symmetricity of the heat diffusion from the recording film in the vertical direction increases, symmetricity for the characteristics between land and groove are increased and, it also had an effect of preventing cross erase which likely to occur most particularly in the groove. Since the substrate deformation occurs upon multi-time rewriting for 500 cycles or more if the thickness of the layer is less than 20 nm, it is preferred that the thickness is 20 nm or more in order to prevent this. While the reflectance is lowered as the recording film is made thinner it can be compensated with the reflectance improving layer. However, when the thickness is further reduced, the reflectance difference between the crystalline state and the amorphous state, the reproducing signal intensity itself is smaller and it can not be thinned further.
(Interface Layer)
In 4.7 GB DVD-RAM, interface layers made of oxide or nitride are disposed on both sides of a recording film (Yasushi Miyauchi, Motoyasu Terao, Akemi Hirotsune, Makoto Miyamaoto, Nobuhiro Tokushuku: Prevention of Inter Diffusion between Protective Layerxe2x80x94Recording Film of Phase Change Optical Disk by Oxide Interface Layer; Pre-text for the Meeting of the Society of Applied Physics, Part 3, 29p-ZK-12 (Spring, 1998) 1127). Both the crystal nuclei forming rate and the crystal growing rate increase compared with the case where (ZnS)80(SiO2)20 protective layers are present on both sides, which increases the crystallization rate. In an example of 4.7 GB DVD-RAM, since a next recording pulse arrives before the solidification after irradiation of one recording pulse by the use of a recording waveform not lowering the power than the erase power level and by reduction of difference for the position between recording pulses adjacent before and after on the recording track by increased density, so that material transfer (flow) of the recording film tends to occur. For improving this, it is effective to reduce the thickness of the recording film thereby relatively enhancing the effect of deposition force to the layers on both sides. However, this lowers both the crystal nuclei forming rate and the crystal growing rate to possibly cause partial erase residue of the amorphous recording mark. However, the worry of occurrence of erase residue is eliminated by the use of both interface layers, for example, made of oxide. Use of nitride is also possible (Mayumi Otowa, Noboru Yamada, Hiroyuki Ohata, Katsumi Kawahara: Phase Change Optical Disk Having Nitride Layers on Both Sides of a Recording Film: Pre-text for the Meeting of the Society of Physics, Third part, 29p-ZK-13 (Spring, 1998) 1128 and N. Yamada, M. Otowa, N. Miyagawa, H. Ohta, N. Akahira and T. Matsunaga: Phase-Change Optical Disk Having a Nitride Interface Layer; Jpn. J. Appl. Phys. Part 1, 37 (1998) 2104)
For the multi-time rewriting, diffusion of Zn, S or the like from upper and lower ZnS.SiO2 protective layers into the recording film has to be prevented. The interface layer also has an effect. In the recording medium, jitter which is a fluctuation of the edge position of the recording mark increases by about 1% at the initial rewriting and the jitter increases or decreases little by little till 1000 cycles of rewriting but this cause no problem at all for the data error. Further, as a result of an acceleration life test, it has been found that the storing life of the recorded data is at least 10 years or more.
(Recording Waveform)
The following relation exists between the recording waveform and the recording mark shape. For example, in 4.7 GB DVD-RAM, since the shortest mark length is 0.42 xcexcm and the linear velocity is 8.2 m/s, a recording pulse for forming one recording mark is divided into two or more. For forming the recording mark exactly, thus, used is a recording waveform in which a portion lowered from the erasing power level is to be decreased or to be absent at all as shown in FIG. 9 while attaching importance to an accurate heating rather than prevention of heat accumulation. Further, as has been already described above, it is also necessary to apply adaptive control for the width of the initial pulse and the last pulse forming the recording mark (adaptive control: an ending position for the last pulse forming the preceding mark and a starting position for the initial pulse forming the succeeding mark are adjusted in accordance with the length of the space to be noted and the length of the preceding mark).
Technique for improving the performance is summarized as below.
1. Technique contributing to narrowing of track pitch
Land/groove recording, absorptivity adjustment, reduction of thickness in the lower protective layer, reduction of thickness in the reflective layer.
2. Technique contributing to narrowing of pit pitch
Mark edge recording, ZCLV recording system, absorptivity adjustment, interface layer, adaptively controlled recording waveform.
3. Technique contributing to increase of speed
1 beam overwrite, recording film composition, absorptivity adjustment, interface layer
As described above, one layer has plural functions and the functions of respective layers are combined in a complicate manner. Reduction in the stress by the reduction of the thickness for the lower protective layer also prevents groove deformation thereby contributing to narrowing of the track pitch. Accordingly, it is extremely important for improving the performance to optimally select the combination and the film thickness of the stacked films.